Analytical instruments are used for a variety of applications generally associated with analyses of materials. One class of such instruments consists of spectrometric instruments, in which a spectrum is generated in interaction with a sample material to effect a spectral beam that is characteristic of the sample and impinged on a photodetector. Modern instruments include a computer that is receptive of spectral data from the detector to generate and compare spectral information associated with the materials. In one type of spectrometric instrument, the spectrum is generated by a dispersion element such as a prism or a holographic grating that spectrally disperses light passed or emitted by a sample or received from a plasma or other excitation source containing sample material. Another type incorporates a time varying optical interference system, in which an interference pattern of light is produced and passed through a sample material that modifies the pattern. Fourier transform computations are applied to the detector signals to transform the modified light pattern into spectral data. The Fourier transform instrument is generally operated in the infrared range, and is known as an "FTIR" instrument.
With improvements in optics, detectors and computerization, there has evolved an ability to perform very precise measurements. Examples are an absorption spectrophotometer, a polychromator or an FTIR instrument that use chemometric mathematical analysis to measure octane number in gasolines. Differences in octane number are associated with subtle differences in near infrared (IR) absorption spectra. The very small changes in spectral characteristics cannot effectively be detected directly by personnel, and computerized automation is a necessity. It also is desirable for such spectral measurements to be effected continuously on-line, typically with an absorption spectrophotometer, whereas an FTIR instrument is often considered a laboratory instrument. Thus there is an interest in utilizing advanced spectrometry methods for analytical chemistry and, more particularly, for precision comparison of information from one type of instrument to that of another.
One aspect of the comparison is that the instruments have intrinsic characteristics that are associated with spectral profiles. Such characteristics are individual to each instrument and may vary with time. Intrinsic characteristics of the instrument distort the data, rendering comparisons inaccurate. In an instrument such as a polychromator with a dispersion grating, an intrinsic characteristic is typified by the profile of spectral data representing a very narrow, sharp spectral line. Such a profile has an intrinsic shape and line width wider than the actual line, due to the fundamental optical design as well as diffraction effects and other imperfections in the optics and (to a lesser extent) electronics in the instrument. An actual intrinsic profile may not be symmetrical. In a polychromator and similar instruments, the instrument profile from a narrow line source is similar to a gaussian profile. For other instruments such as FTIR the intrinsic profile at the limit of interferometer resolution is more rectangular.
U.S. Pat. No. 5,303,165 (Ganz et al), commonly owned by the present assignee, discloses a method and apparatus for standardizing a spectrometric instrument having a characteristic intrinsic profile of spectral line shape for a hypothetically thin spectral line in a selected spectral range. Standardized data is substantially the same as that obtained from the same sample material with any similar instrument, and repeatedly with the same instrument over time.
Conventional FTIR instruments are taught in textbooks such as "Fourier Transform Infrared Spectrometry" by P. R. Griffiths and J. A. de Haseth. In these instruments, an interference pattern of light is produced with a Michaelson or similar interferometer comprising a beam splitter which is a partial reflector that splits white light into two beams. These beams are reflected back and recombined at the beam splitter. The path length of one of the beams is varied with time to produce a time-varied interference pattern. This light pattern is directed through a sample material that modifies the pattern. Fourier transform computations transform the modified pattern into spectral data representing intensity vs. wavenumber.
Other classes of analytical instruments are for chromatography, of which there are two common classes, namely gas chromatography (GC) and liquid chromatography (LC). Gas chromatography, for example as illustrated in U.S. Pat. No. 5,545,252 and U.S. Provisional Patent Application No. 60/006,017 is essentially a physical method of separation in which constituents of a test sample in a carrier gas are adsorbed and desorbed by a stationary phase material in a column. A pulse of the sample is injected into a steady flow of carrier gas. At the end of the column the individual components are separated by time in varying degrees. Detection of a gas property such as thermal conductivity provides a time-scaled pattern which, by calibration or comparison with known samples, indicates the constituents of the test sample qualitatively and quantitatively. The main components of such a system are the column, an injector with a mixing chamber for introducing the sample into the carrier gas, a gas-property detector at the outer end of the column, gas controls and a computer for treating and displaying the output of the detector. Comparisons are of interest between GC instruments having different types of columns or different operating conditions.
Liquid chromatography is similar, for example as illustrated in U.S. Pat. No. 4,886,356 (Paradis) and U.S. Pat. No. 5,173,742 (Young). A pulse of sample is injected into a steady flow of a carrier liquid which is passed through a column and thence through a cell (or two cells) with windows. A light beam is passed through each cell to a photodetector that generates an output signal to a computer, the signal varying with time according to the sample constituents. As with gas chromatography, by computer treatment and calibration or comparison with known samples, the constituents of the test sample may be indicated. Comparisons are of interest between LC instruments having different types of columns or different operating conditions.
Objects of the invention are to provide a novel method and a novel means for transforming analytical information of one of analytical instrument for comparison with spectral information of another type of spectrometric instrument, where the instruments may be different types within the same class of instrument or the same type with different operating conditions. Such objects include providing a novel method and a novel means for transforming spectral information of one spectrometric instrument for comparison with spectral information of another spectrometric instrument. Particular objects are to provide a method and a means for transforming spectral information of an FTIR type of spectrometric instrument for comparison with spectral information of a dispersion type of spectrometric instrument, more particularly where the spectral information of the dispersion type of spectrometric instrument is standardized. Further objects are to provide a novel method and a novel means for transforming analytical information of one chromatographic instrument for comparison with spectral information of another chromatographic instrument of the same class as the first, viz. GC or LC.